1. Field of the Invention
The present invention relates to a lightweight, easy to use wind tunnel for demonstrating principles of aerodynamics in an educational setting. Overall, the wind tunnel and its components are quiet, accurate, and easy to use.
2. Brief Description of the Prior Art
Full size wind tunnels have been used for years to test the aerodynamics of automobiles and airfoils (wings). As people have become more environmentally conscious, designers of vehicles have once again turned their focus toward fuel conservation. One way of increasing the fuel efficiency of a vehicle is to streamline its shape. A streamlined shaped poses less resistance to advancing air flow, allowing the vehicle to advance through the air at a given rate with greater efficiency. Vehicle designers and engineers, seeking to streamline a vehicle silhouette rely on both theoretical aerodynamics and empirical wind tunnel testing.
Aerodynamics is generally introduced in junior and senior high school science and technology classes, with a greater technical concentration coming in the later grades and college. In conjunction with a high school physics program, students learn the fundamentals of lift and drag. Lift is the component of the total aerodynamic force acting on a airfoil perpendicular to the relative wind and normally exerted in a upward direction, opposing the pull of gravity. Drag is the retarding force exerted on a moving body, such as an airfoil, by a fluid medium such as air or water. Students also learn about calculating the coefficient of drag, or C.sub.D, which represents the ratio of the drag force to the dynamic pressure.
Generally, wind tunnels require a considerable investment. In addition to the financial resources required, a typical wind tunnel will also require significant periods of time to begin and maintain an extensive correlation program. Such correlation is required to ensure that the results obtained during testing will be meaningful. Some wind tunnels even require a dedicated staff to prepare and maintain the tunnel for testing. Consequently, someone desiring to use a wind tunnel for testing can anticipate incurring considerable expense and delay.
Typical wind tunnels share several common design aspects. In general, an air stream generated by a fan or blower passes through a constrictor, a test section, a diffuser, and then exits the tunnel. The fan or blower may be attached to either end of the tunnel. In an in-draft wind tunnel, the fan or blower connects to the tunnel downstream of the diffuser. When the fan or blower is activated, the air stream is pulled or sucked through the tunnel. In contrast, the air stream is pushed through a wind tunnel of the blower type. In a blower tunnel, the fan or blower attaches to the tunnel upstream of the constrictor end.
Either type of wind tunnel usually features a flow straightener attached to the tunnel directly downstream of the air inlet. In the case of an in-draft tunnel, the air is pulled into the tunnel at the constrictor end, so the flow straightener is attached to the tunnel at that point. Flow straighteners are necessary because the air around the inlet enters from all directions, not just a direction normal to the inlet. In some wind tunnels, the air stream also passes through a screen or screens. A screen helps create a Uniform velocity profile and reduce the turbulence level of the entering airstream.
The constrictor and diffuser serve related but opposing functions in a wind tunnel: the constrictor reduces the cross-sectional area available to the air stream; the diffuser increases the area available to the air stream. In conjunction, the constrictor and diffuser create a pressure drop across the tunnel, causing the air stream velocity to increase as it moves from an area of high pressure at the constrictor end across the test section to an area of low pressure at the diffuser end. The constrictor serves the added function of reducing turbulence in the entering air stream. Turbulence downstream of the constrictor is reduced directly proportional to the change in the cross-sectional area before and after the constrictor. For example, if the constrictor reduces the cross-sectional area from 4 in.sup.2 to 2 in.sup.2, the turbulence downstream of the constrictor will be halved In appearance, the design of a constrictor or a diffuser can resemble a funnel. A blower wind tunnel can be operated without a diffuser, but the required horsepower for the fan or blower would increase dramatically.
In operation, investigators often desire to gain a more tangible appreciation for the characteristics of the flow around a particular area of a body. To do so, a user will often probe the air stream using a tuft wand. A tuft wand is usually a thin rod with a strip of lightweight, flexible material, such as a piece of yarn, attached to the probing end. Because the rod is relatively thin, the user may introduce the tuft wand into the tunnel air stream unobtrusively by passing the probing end through a small hole upstream of the body being tested. By placing the probing end near the surface of the body being tested at a particular point, the user can understand the characteristics of the flow at ,that point. If the flexible material flutters slightly in a well-behaved manner, the flow is characterized as attached. If the flexible material flaps wildly and unpredictably, however, the flow at that point is unattached.
With respect to demonstrating the general aerodynamic fundamentals introduced above an inexpensive and far less complicated wind tunnel design than those that are currently used for full-scale or research testing will suffice. Such a wind tunnel design, suited for giving classroom demonstrations to high school students, would allow the instructor to show, e.g., how the lift and drag forces on an airfoil interact and change as the angle of attack increases. Such a design would be suited for low-end mass production, in contrast to the one-off designs used by research labs. Students could also use the wind tunnel for taking accurate wind resistance measurements and identifying the areas of greatest resistance on a body subjected to an air stream, e.g., by use of a tuft wand. One disadvantage of the previous attempts at constructing a wind tunnel suitable for demonstrations, however, is that the high noise levels resulting from operating the fan prohibited conversation between the instructor and the audience. Another drawback of earlier designs concerned their general inaccuracy and imprecision.
By limiting the goal of this design to one of demonstrating fundamentals, several benefits result. The design of the present invention is safe and simple to operate, relatively insensitive to mishandling, lightweight, inexpensive, and portable.